The invention concerns a method of treading vulcanized tire carcasses. These methods are generally applied today in retreading, but, of course, everything to be stated below is applicable to the treading of new tire carcasses.
There are two main types of retreading methods, one, commonly called xe2x80x9ccold retreading,xe2x80x9d consisting of using a precured tread that is placed on a carcass ready to be retreaded, and then curing the assembly by using heating means such as an oven; the other method, called xe2x80x9chot retreading,xe2x80x9d consists of placing a crude rubber tread, notably, in the form of sheets, strips or sections, on a carcass ready to be retreaded and then curing the assembly in a mold.
These hot retreading methods are applied, in particular, to the retreading of large-sized tires, such as tires intended for trucks, farm machinery, construction equipment, etc., for which problems of handling and warping of treads render their use difficult in a precured form. The invention lies within the area of hot retreading.
Among the hot retreading methods, U.S. Pat. No. 4,053,265 describes a method which consists of using a resilient ring mold, preferably made of rubber, designed to make possible the molding and vulcanization of the tread. This resilient mold is placed on the tire carcass to be retreaded, which has been previously covered with a crude rubber tread, the mold-carcass assembly then being totally enveloped by a membrane ensuring tightness. Said assembly is then placed in an oven for curing after creation of a vacuum in the membrane.
More precisely, in order to ensure perfect tightness between the carcass, the tread and the resilient mold, as shown in U.S. Pat. No. 4,053,265 (FIG. 2), it is necessary to form the casing of the carcass covered by the mold by means of two resilient membranes: one so-called xe2x80x9cinnerxe2x80x9d membrane which extends from the outer surface of the sidewalls over the entire inner surface of the carcass and a so-called xe2x80x9couterxe2x80x9d casing which is covered by the first membrane at the outer surface of the sidewalls, or inversely, in order to guarantee the continuity of tightness between the two membranes, that outer membrane thus covers the whole outer surface of the mold and the part of the sidewalls not covered by the inner membrane.
The placement of those membranes is particularly difficult. In fact, it is necessary to stretch the outer membrane very extensively so that it can cover all of the tread and sidewalls, that operation being rendered even more difficult by the fact that large-sized tires are generally involved. This problem further entails very rapid wear of that outer membrane subjected to severe elastic stresses and a tendency to tear, this resulting in the need to change those membranes frequently to avoid the risks of using a damaged membrane causing defects in the manufactured tire.
U.S. Pat. No. 4,053,265 proposes an alternative to the use of these two membranes, which consists of using an outer membrane identical to that of the first solution, except that it employs an O-ring to form the seal at the sidewalls and a membrane inside the tire in the form of a tube connected to a tire casing support disk. It is clear that this solution does not solve the aforesaid problems associated with use of the outer membrane and that, furthermore, this solution makes it possible to create a good seal only on condition of vulcanizing the O-rings on the sidewalls of the tire carcass, which is problematical.
The object of the invention is a method of hot treading using a resilient ring mold which makes it possible to overcome these difficulties.
According to the invention, the method comprises the following steps:
depositing a crude rubber tread on a tire carcass to be treaded;
mounting the tire carcass thus covered on a rim, positioning on the tread a resilient ring mold possessing an inner annular surface ensuring molding of the tread and inflating the tire carcass;
placing a resilient flange between each bead and each peripheral edge of the mold, covering the outer surface of the tire casing lying between said bead and said edge; and
placing the assembly in a pressurized and heated enclosure.
The invention also concerns treading apparatus including a support rim for said carcass, a resilient ring mold for molding a crude rubber tread covering the tire carcass and two resilient flanges to cover the outer surface of the tire casing thus formed, that is, the surface lying between a bead of said casing and the peripheral edge of the corresponding mold.
The choice of radial and axial directions for the retreading means as well as for the tire relates below to the axes generally used as reference in the tire, the axial direction corresponding to a direction parallel to the axis of rotation of the tire casing, and the radial planes being planes containing a radius of the tire casing and passing through said axis of rotation.
Other characteristics and advantages of the invention will appear on reading a working example of a retreading means according to the invention, with reference to the attached drawings.